The word cement is used here in its wider sense, and thus comprises, in addition to Portland cement, hydraulic binding agents such as aluminate cement, slag cement etc.
According to the information sheet B8:1973 published by "Byggforskningen" ("Construction Research") and entitled "Betongtillsatsmedel" ("Concrete Additives") hitherto known concrete additives can be divided up into a plurality of different groups, of which the first two are "Luftporbildande tillsatsmedel" ("Air-entraining additives") and "Vattenreducerande (plasticerande) tillsatsmedel" ("Water-reducing (plasticizing) additives").
The present invention relates to these two groups, although it can still not be referred entirely to one or the other or to both.
A fresh cement-based binding agent mixture (cement mortar or concrete mass) consists of solid particles, water and air. The cement-bonded concrete, which from the point of view of volume has the largest share within the construction industry, substantially consists of approx. 100 liters of cement, 200 liters of water, 650 liters of stone material, all of which with a diameter of less than 4 mm is usually designated sand, and the remainder stone, and 50 liters of air, counted on 1000 liters of fresh concrete mass. Of the 200 liters of water which is required in order to make it possible to process the mixture, approx. 60 liters is bonded chemically in the hardened cement paste, while the remaining quantity is bonded physically as gel and capillary water.
The solid particles comprised in the cement mortar or the concrete consist of aggregate, i.e. stones and sand of various fractions, the actual cement grains, and hydratation products precipitated in water. The cement grains react with parts of the water mixture to form a hydratation product which consists of a colloidal glue, the cement gel. The remaining water and the air are distributed in the basic mass formed by cement gel and aggregate. In the fresh mortar, the water will be found in the form of menisci in the cavities between solid cement and aggregate particles in their vicinity, while the air, in turn, forms pores between these particles and the water menisci. The particle size of the previously mentioned precipitated hydration products is within the Angstrom range, while the mean grain size of the cement grains is approx. 5 .mu.m. The sand and other aggregate material, finally, can have a particle size from approx. 0.1 mm up to one or a few centimeters. If no special measures are taken, a fresh cement mortar will have an air content of between 1.5 and 3.5 percent by volume. In the hardened cement-bonded mass, there are both air and water-filled pores. In addition to these pores, the size of which in a well packed cement-bonded mass is between 10.sup.-1 and 1 mm, also so-called capillary pores are formed, with a pore size of 10.sup.-4 to 10.sup.-2 mm and in the hardened cement gel so-called gel pores with a pore size of approx. 10.sup.-6 mm.
The size and quantity of the gel pores can be influenced only to a little extent via the water content of the original mixture, while on the other hand, the capillary pores are determined by the water cement ratio. A great many different ways of increasing the air pore content in fresh cement or concrete mass is described in literature.
In the Construction Research Brochure it is stated that such air entraining agents increase the total air content in the fresh cement or concrete mixture, and also cooperate towards a more uniform distribution of the air pores in the basic mass, at the same time as, to a certain extent, one obtains an increase of the content of small air bubbles, i.e. bubbles with a diameter of between 0.05 and 0.5 mm. As long as these finely distributed air bubbles exist, this gives the fresh mass an improved stability, which also contributes towards less water separation. If it is primarily desired to improve the stability of the fresh mass, without any requirements other than a certain air content, according to generally known technology, it is sufficient to dose for an air content of 3.0-4.0%. An increased admixture of air also has a certain improving effect on the flow of the fresh mass, as the air pores give rise to less friction between the solid particles in the mass, and thereby make this easier to work with. However, high contents of solid fine material at an increased air content are considered to give a tough, sticky concrete. As the consistency of a cement or concrete mass as a rule is used as a basis, the water content of the mixture can usually be lowered by an admixture of an air-entraining agent. According to a rule of thumb given in literature, it should be possible to reduce the water content in fresh cement mortar, with unchanged consistency, by one-half of the air content increase achieved through the addition of an air-entraining agent. Together with the previously mentioned reduced water separation, an increase of the quantity of fine air pores in the basic mass also involves that large aggregate particles are not as easily separated out of the fresh mixture. However, the changes in consistency hereby achieved are comparatively limited, as they are directly dependent on the quantity of stable air which in this way can be drawn into the mass. However, the perhaps most common reason for adding air-entraining agent is that it is desired to make the hardened mass more resistant to frost, since the cavities achieved by the addition of air-entraining agent will be available as expansion chambers for other water existing in the pore system when this increases its volume in connection with its freezing. The walls of the pores are hereby prevented from being broken when the ambient temperature falls below the feezing point. An air pore volume of approx. 5 percent by volume is considered to give a maximum resistance to frost, and this can comparatively easily be achieved. As long as the strength of any aggregate material is greater than that of the stiffened cement paste, the strength of this will determine the strength of the mass. The properties of the hardened mass will to a very great exgent be dependent on the water and air content of the original mixture. A plurality of different materials has been used as air-entraining agents, such as saponified resins, alkyl aryl sulfonate, calcium ligno sulfonate and hydroxy ethyl cellulose, in combination with tensides. From the point of view of functioning, these additives are based upon the fact that with the aid of the foaming agents comprised in them, a more or less stable foam is built up, with the aid of which increased quantities of air can be introduced into a fresh cement or concrete mass. The air pores hereby initiated will substantially be of the magnitude of 0.1-1 mm. These additives make it possible to manufacture cement mortar and concrete with a reduced density. However, foam bubbles of this size have poor own strength, and the pore system hereby built up can therefore collapse before the cement bonding agent has had time to harden. This applies particularly when it is desired to introduce large quantities of air. The mainly hydrophilic nature of the additives can also contribute towards an increased water absorption in the hardened mass. Through the addition of only tensile (either anion active or non-ion active) it is also possible, within certain limits, to change both the consistency and the quantity of the air comprised in a fresh cement composition. However, regardless of the type of tenside which has been used, this procedure has proved to be very sensitive as regards the quantity of tenside added, which at the most should comprise one or a few per mille of the entire mixture. The tensides used in this connection are highly effective, and can rapidly give a great quantity of air bubbles. However, the stability of these varies considerably. As a rule, anion active tensides lower the surface tension drastically when small quantities are added, while the non-ion active tensides have a somewhat lesser effect with one and the same concentration. With these two types of tensides, however, particularly at an over-dosing, the air bubbles generated from the beginning are rapidly recombined, i.e. they join together to form larger units. Particularly with the anion-active tensides, this recombination can take place to such an extent that air leaves the system, and a collapse occurs, i.e. the fresh mixture shrinks. Certain non-ion active tensides show considerably better stability, and therefore a greater tolerance towards over-dosing, but it is very noticeable, however, that recombination increases at e.g. more intensive stirring. Nor is it possible through regulating of such parameters as the choice of type of stirrer, the quantity of tenside added, and the intensity of the stirring, to control the quantity of air mixed in or the size of the air pores, which will vary between 0.1 and several mm.
When additives of the kind described above are used, the intention can be to mix in air, or that it is desired not to add more air to a concrete mixture. Through the choice of type of tenside and the quantity added, both of these effects can be achieved. In the Swedish published application No. 333 113, it is described how, through the addition of various tensides plus a styrene acrylate dispersion, the workability and flow of a concrete mixture increases. As this addition permits a considerable reduction of the water cement ratio of the fresh mixture, the hardened concrete mixture can be given a more compact structure and, consequently, increased strength. It is said that the dispersion in question, notwithstanding a high content of tensides, does not have any foaming capability. It is also particularly pointed out that it does not give rise to any formation of air pores. However, the quantity of tensides added and the quantity of acrylonitrile comprised in the polymer will make the hardened concrete highly hydrophilic.
In the Swiss patents Nos. 493,438 and 515,862 cement and concrete additives are described consisting of polymer or natural latex dispersions containing water, to which in addition to polymer components and emulsifiers, also an anti-foaming agent has been added.
Further, in the U.S. Pat. No. 3,819,391, an air-entraining cement additive has been described, consisting of a free-flowing flaky solid product containing 12.5-37.5 percent by weight of a bituminous substance and the remainder, 87.5-62.5 percent by weight, of a surface-active substance. In this additive, the major portion thus consists of the surface-active substance.
The Swedish patent application 7600161-9 relates to a development of the additive according to the above-mentioned U.S. patent, here in the form of a powder-formed product, soluble in water, which to 40-60 percent by weight is built up of the above-mentioned bituminous substance and a surface-active substance, while the remaining 60-40 percent by weight consist of polyethylene oxide resins, lignol sulfonates and diatomaceous earth. As a surface-active agent it is said that both anion, cation and non-ion agents can be used, but that a mixture is preferred. It is stated that the bituminous material may be asphalt, coal tar or derivatives thereof. In order that it may be used in this connection, however, it is a requirement that the substance in question shall be a liquid at room temperature. In addition to its air-entraining function, it is said that the additive also has a binding-retarding effect on the cement.
In the Swedish patent application No. 74.03454-7, it is also shown how, with the aid of colloidal silica, surface-active substances and amphiphilic substances or hydrocarbons, the consistency, workability and uniform distribution of the fine portion of the cement can be changed. In a table on page 6, the great importance of the water cement ratio is shown. When, with the aid of an additive, more air is introduced into the concrete, the water content can be reduced at the same time. The greatest reason for the increase in strength reported in the application must presumably be ascribed to the reduced water content. However, it should be possible to refer a complementary effect to the silica which is chemically active in connection with the hardening of the cement.
As indicated by the above-mentioned review of at least some of the concrete additives which have previously been proposed, there is nothing new in endeavouring to manipulate the structure of a fresh cement composition through miscellaneous additives, which primarily have an, albeit limited, effect of drawing in air. The fact that at least some of these air-entraining agents have also had a tendency to increase the content of fine air pores in the mixture is likewise previously known. In general, however, these older types of air-entraining agents have also given rise to large quantities of comparatively large pores, i.e. pores with a size of 0.1-1.0 mm and more.